Alkoxysilalkylene terminated siloxanes are used to prepare room temperature vulcanizing (RTV) silicone sealants which employ titanate catalysts to facilitate cure upon exposure of the sealant to atmospheric moisture. U.S. Pat. No. 4,772,675 teaches that RTV silicone sealants prepared with such alkoxysilalkylene terminated siloxane polymers have greatly improved shelf-life compared to similar sealants prepared from more conventional alkoxysilyl terminated siloxanes, in which the terminal group is bonded to the next silicon through an oxygen link.
Weyenberg teaches in U.S. Pat. No. 3,175,993, issued Mar. 30, 1965, organopolysiloxanes having terminal alkoxylated silcarbane groups. Alkoxylated silcarbane groups are equivalent to alkoxysilalkylene groups. A method of preparation is shown which reacts an SiH endblocked polydiorganosiloxane with an alkoxy silane having an aliphatically unsaturated group in the presence of a platinum catalyst. However, this patent does not disclose a method for conveniently obtaining the required SiH endblocked polydiorganosiloxane precursor.
More recently, Lucas teaches in U.S. Pat. No. 4,599,394 the preparation of these polymers by the platinum catalyzed addition of an alkoxy-functional SiH compound with vinyl-ended polydimethylsiloxane.
There are some significant limitations to the practical use of the Weyenberg and Lucas approaches of making these polymers. The Weyenberg approach teaches the use of silicon-hydride-ended polymers, which are not commonly available. An economical method of producing these precursors is not given. The Lucas approach requires the use of trimethoxy- or triethoxy silane to cap the silicon-vinyl ended polymers and these silanes have significant toxicity concerns.
Compared to silicon-hydride-ended polymers or silicon-vinyl-ended polymers, silanol-ended polymers are cheaper and more readily available. Silanol-ended polymers are the basis for most of the current sealant products. It is, therefore, of interest to find a route to polymers endblocked with alkoxysilalkylene groups, starting from the commercially available silanol-ended polymers.
One such route was discovered by Krahnke and Saam and disclosed in U.S. Pat. Nos. 4,847,400 and 5,017,672. In this single step route, a silanol polymer is capped with a silazane having the trialkoxysilylethylene linkage, ((MeO).sub.3 SiC.sub.2 H.sub.4 Me.sub.2 Si).sub.2 NH, in the presence of an acid catalyst, such as acetic acid (HOAc), trifluoroacetic acid, or dodecylbenzenesulfonic acid. Although all these catalysts were capable of producing a polymer that could be used in a sealant, each had its disadvantages. Acetic acid is inefficient and requires high concentrations and long times to complete capping. Dodecylbenzenesulfonic acid is an efficient catalyst, but reacts with ammonia to give ammonium dodecylbenzene sulfonate, and the resulting polymers are cloudy due to the incompatibility. Trifluoroacetic acid is the catalyst of choice to produce a clear polymer efficiently.
Development work has shown, however, that a significant amount of condensation occurs between the silanol end of the polymer and the alkoxy group of the silazane endcapper to cause chain extension and an increase in viscosity during the capping reaction. This viscosity increase may reduce the extrusion rate of the compounded sealant below acceptable limits.
Bilgrien et. al. teach another route in U.S. Pat. No. 4,962,174. An alkoxysilethylene endblocked polydiorganosiloxane is produced by first reacting hydroxyl endblocked polydiorganosiloxane with excess SiH compound having from 2 to 4 SiH groups, in the presence of a platinum catalyst, then reacting that product with vinyltrialkoxysilane to give the desired polymer. The SiH compounds include silanes, disilanes, and disiloxanes, and the preferred excess is 10 moles of SiH for each mole of SiOH present. This process is somewhat inefficient, as demonstrated by the large excess of SiH compound required. Further, some loss in sealant properties is incurred on shelf aging, due to incomplete conversion of SiOH, even with the excess SiH reagent.